Re: [PATCH v12 06/46] arm64: RMI: Define the user ABI

[Marc Zyngier <maz@kernel.org>]

On Wed, 17 Dec 2025 10:10:43 +0000,
Steven Price <steven.price@arm.com> wrote:
> 
> There is one CAP which identified the presence of CCA, and two ioctls.
> One ioctl is used to populate memory and the other is used when user
> space is providing the PSCI implementation to identify the target of the
> operation.
> 
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v11:
>  * Completely reworked to be more implicit. Rather than having explicit
>    CAP operations to progress the realm construction these operations
>    are done when needed (on populating and on first vCPU run).
>  * Populate and PSCI complete are promoted to proper ioctls.
> Changes since v10:
>  * Rename symbols from RME to RMI.
> Changes since v9:
>  * Improvements to documentation.
>  * Bump the magic number for KVM_CAP_ARM_RME to avoid conflicts.
> Changes since v8:
>  * Minor improvements to documentation following review.
>  * Bump the magic numbers to avoid conflicts.
> Changes since v7:
>  * Add documentation of new ioctls
>  * Bump the magic numbers to avoid conflicts
> Changes since v6:
>  * Rename some of the symbols to make their usage clearer and avoid
>    repetition.
> Changes from v5:
>  * Actually expose the new VCPU capability (KVM_ARM_VCPU_REC) by bumping
>    KVM_VCPU_MAX_FEATURES - note this also exposes KVM_ARM_VCPU_HAS_EL2!
> ---
>  Documentation/virt/kvm/api.rst | 57 ++++++++++++++++++++++++++++++++++
>  include/uapi/linux/kvm.h       | 23 ++++++++++++++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 01a3abef8abb..2d5dc7e48954 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6517,6 +6517,54 @@ the capability to be present.
>  
>  `flags` must currently be zero.
>  
> +4.144 KVM_ARM_VCPU_RMI_PSCI_COMPLETE
> +------------------------------------
> +
> +:Capability: KVM_CAP_ARM_RMI
> +:Architectures: arm64
> +:Type: vcpu ioctl
> +:Parameters: struct kvm_arm_rmi_psci_complete (in)
> +:Returns: 0 if successful, < 0 on error
> +
> +::
> +
> +  struct kvm_arm_rmi_psci_complete {
> +	__u64 target_mpidr;
> +	__u32 psci_status;
> +	__u32 padding[3];
> +  };
> +
> +Where PSCI functions are handled by user space, the RMM needs to be informed of
> +the target of the operation using `target_mpidr`, along with the status
> +(`psci_status`). The RMM v1.0 specification defines two functions that require
> +this call: PSCI_CPU_ON and PSCI_AFFINITY_INFO.
> +
> +If the kernel is handling PSCI then this is done automatically and the VMM
> +doesn't need to call this ioctl.

Shouldn't we make handling of PSCI mandatory for VMMs that deal with
CCA? I suspect it would simplify the implementation significantly.

What vcpu fd does this apply to? The vcpu calling the PSCI function?
Or the target? This is pretty important for PSCI_ON. My guess is that
this is setting the return value for the caller?

Assuming this is indeed for the caller, why do we have a different
flow from anything else that returns a result from a hypercall?

> +
> +4.145 KVM_ARM_RMI_POPULATE
> +--------------------------
> +
> +:Capability: KVM_CAP_ARM_RMI
> +:Architectures: arm64
> +:Type: vm ioctl
> +:Parameters: struct kvm_arm_rmi_populate (in)
> +:Returns: number of bytes populated, < 0 on error
> +
> +::
> +
> +  struct kvm_arm_rmi_populate {
> +	__u64 base;
> +	__u64 size;
> +	__u64 source_uaddr;
> +	__u32 flags;
> +	__u32 reserved;
> +  };
> +
> +Populate a region of protected address space by copying the data from the user
> +space pointer provided. This is only valid before any VCPUs have been run.
> +The ioctl might not populate the entire region and user space may have to
> +repeatedly call it (with updated pointers) to populate the entire region.

size as a __u64 is odd, as the return value from the ioctl is a signed
int. This implies that you can't really report how many bytes you have
copied.  Some form of consistency wouldn't hurt.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.